The present disclosure relates to improvements to and novel applications for the LDA. The LDA was first described in U.S. Pat. No. 7,911,235.
The LDA of the present disclosure is similar in some respects to super-regenerative receivers (SRO) in terms of circuit topology. SROs are amplitude sensitive regeneration devices. SROs also have external quenching and high gain. The SRO was first described in U.S. Pat. No. 1,424,065. SROs typically suffer from poor selectivity and higher output noise when used for narrow band signals. SROs also may drift in temperature when the oscillator is LC based. The SRO receiver was quickly replaced by the super-heterodyne receiver for mainstream radio, because the latter has superior selectivity and sensitivity. However, the SRO is simple and low power, and has been used over many decades for remote control systems, short distance telemetry, and wireless security. Selectivity and drift limitations have been addressed through the use of surface acoustic wave (SAW) devices. In the first decade of the 21st century, there has been a renewed interest in SROs for use in low power receivers up to the GHz range, and for moderate to high data rate applications.
The receive sensitivity of an SRO at 1 MHz bandwidth is in the medium to high range, in the order of −80 dBm to −90 dBm. The dynamic range (minimum to maximum signal level range) of an SRO is medium, in the order of 20 to 60 dB. SROs are not able to demodulate phase modulation (PM) intrinsically or otherwise. SROs are not able to reduce noise. SROs can be placed anywhere in the receive chain, but with a loss of receive sensitivity, unless placed upfront. SROs are externally quenched (or synchronized). The amplification mode of an SRO is amplitude sensitive regeneration. The circuit topology is generally Colpitt oscillator-based. The gain of an SRO is high.
The present disclosure also has some similarities to DC or baseband log amps, which tend to provide logarithmic amplification over a wide dynamic range. Baseband log amps are based on multiple Gilbert cells, and typically provide a good linearity over a mid to large dynamic range at low to high frequencies. Simpler logarithmic amplifiers (e.g., DC log amps) are based on transistor logarithmic current versus voltage transfer characteristics, and address applications ranging from DC to low frequency.
The receive sensitivity of log amps at 1 MHz bandwidth is in the medium to high range, in the order of −80 dBm to −90 dBm. The dynamic range (minimum to maximum signal level range) of a log amp is high, in the order of 40 to 90 dB. Log amps are not able to demodulate PM directly or indirectly. Log amps are not able to reduce noise. Log amps are not used in the receive chain and do not involve quenching. The amplification mode of log amp is multiple amplification. The circuit topology is typically multi-stage Gilbert cells. The gain of a log amp is high to very high, in the order of 30 to 70 dB.
Hence, neither SROs nor log amps have the ability to intrinsically demodulate phase, amplitude and phase, frequency and amplitude and frequency with high skirt ratio, very high sensitivity and noise suppression, very high dynamic range, superior discrimination, and flexible placement in a receiver chain without drawback.
Additional methods have been developed to process a weak signal buried in noise, such as averaging, selective amplification, filtering, synchronized detection, spread spectrum and nonlinear RAMAN optic amplifier.
In averaging, noise is reduced over n periods; however the signal is not amplified. Also averaging requires an accurate trigger for reference, and this trigger may be noisy and problematic at low signal levels.
In selective amplification and/or filtering, the amplification and/or filtering are frequency dependent and stationary, so they do not provide any improvement over time in the frequency pass band, nor do they reduce the noise in that pass band. This is problematic if the bandwidth is large. Also, selective amplifiers have a limited noise rejection.
In synchronized detection, a phase lock loop (PLL) is required to lock it to the input signal, which selectivity implies a narrow band unless more complicated methods are used. This method may also be problematic at very low signal levels.
In Direct Sequence-Spread Spectrum (DS-SS), bits are spread over a wide frequency spectrum during the transmitting modulation process, and are eventually communicated over a lossy medium. The receiver dispreads energy and makes the demodulated signal appear much above the noise floor (e.g., GPS with a typical spreading factor of one thousand). This methods allows very high attenuation to be overcome, but this method requires a DS-SS transmitter that is not practical for many applications.
In a RAMAN distributed optic amplifier, the Signal-to-Noise-Ratio (SNR) can be improved and data can be transported through fiber optic lines over hundreds or thousands of kilometers with only minimum regeneration, but the solution is limited to optic applications.